powerpc: Revert "Use the POWER8 Micro Partition Prefetch Engine in KVM HV on POWER8"

This reverts commit 9678cdaa ("Use the POWER8 Micro Partition
Prefetch Engine in KVM HV on POWER8") because the original commit had
multiple, partly self-cancelling bugs, that could cause occasional
memory corruption.

In fact the logmpp instruction was incorrectly using register r0 as the
source of the buffer address and operation code, and depending on what
was in r0, it would either do nothing or corrupt the 64k page pointed to
by r0.

The logmpp instruction encoding and the operation code definitions could
be corrected, but then there is the problem that there is no clearly
defined way to know when the hardware has finished writing to the
buffer.

The original commit attempted to work around this by aborting the
write-out before starting the prefetch, but this is ineffective in the
case where the virtual core is now executing on a different physical
core from the one where the write-out was initiated.

These problems plus advice from the hardware designers not to use the
function (since the measured performance improvement from using the
feature was actually mostly negative), mean that reverting the code is
the best option.

Fixes: 9678cdaa ("Use the POWER8 Micro Partition Prefetch Engine in KVM HV on POWER8")
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/cache.h

@@ -3,7 +3,6 @@
 
 #ifdef __KERNEL__
 
-#include <asm/reg.h>
 
 /* bytes per L1 cache line */
 #if defined(CONFIG_8xx) || defined(CONFIG_403GCX)
@@ -40,12 +39,6 @@ struct ppc64_caches {
 };
 
 extern struct ppc64_caches ppc64_caches;
-
-static inline void logmpp(u64 x)
-{
-	asm volatile(PPC_LOGMPP(R1) : : "r" (x));
-}
-
 #endif /* __powerpc64__ && ! __ASSEMBLY__ */
 
 #if defined(__ASSEMBLY__)

arch/powerpc/include/asm/kvm_host.h

@@ -297,8 +297,6 @@ struct kvmppc_vcore {
 	u32 arch_compat;
 	ulong pcr;
 	ulong dpdes;		/* doorbell state (POWER8) */
-	void *mpp_buffer; /* Micro Partition Prefetch buffer */
-	bool mpp_buffer_is_valid;
 	ulong conferring_threads;
 };
 

arch/powerpc/include/asm/ppc-opcode.h

@@ -141,7 +141,6 @@
 #define PPC_INST_ISEL			0x7c00001e
 #define PPC_INST_ISEL_MASK		0xfc00003e
 #define PPC_INST_LDARX			0x7c0000a8
-#define PPC_INST_LOGMPP			0x7c0007e4
 #define PPC_INST_LSWI			0x7c0004aa
 #define PPC_INST_LSWX			0x7c00042a
 #define PPC_INST_LWARX			0x7c000028
@@ -285,20 +284,6 @@
 #define __PPC_EH(eh)	0
 #endif
 
-/* POWER8 Micro Partition Prefetch (MPP) parameters */
-/* Address mask is common for LOGMPP instruction and MPPR SPR */
-#define PPC_MPPE_ADDRESS_MASK 0xffffffffc000ULL
-
-/* Bits 60 and 61 of MPP SPR should be set to one of the following */
-/* Aborting the fetch is indeed setting 00 in the table size bits */
-#define PPC_MPPR_FETCH_ABORT (0x0ULL << 60)
-#define PPC_MPPR_FETCH_WHOLE_TABLE (0x2ULL << 60)
-
-/* Bits 54 and 55 of register for LOGMPP instruction should be set to: */
-#define PPC_LOGMPP_LOG_L2 (0x02ULL << 54)
-#define PPC_LOGMPP_LOG_L2L3 (0x01ULL << 54)
-#define PPC_LOGMPP_LOG_ABORT (0x03ULL << 54)
-
 /* Deal with instructions that older assemblers aren't aware of */
 #define	PPC_DCBAL(a, b)		stringify_in_c(.long PPC_INST_DCBAL | \
 					__PPC_RA(a) | __PPC_RB(b))
@@ -307,8 +292,6 @@
 #define PPC_LDARX(t, a, b, eh)	stringify_in_c(.long PPC_INST_LDARX | \
 					___PPC_RT(t) | ___PPC_RA(a) | \
 					___PPC_RB(b) | __PPC_EH(eh))
-#define PPC_LOGMPP(b)		stringify_in_c(.long PPC_INST_LOGMPP | \
-					__PPC_RB(b))
 #define PPC_LWARX(t, a, b, eh)	stringify_in_c(.long PPC_INST_LWARX | \
 					___PPC_RT(t) | ___PPC_RA(a) | \
 					___PPC_RB(b) | __PPC_EH(eh))

arch/powerpc/include/asm/reg.h

@@ -226,7 +226,6 @@
 #define   CTRL_TE	0x00c00000	/* thread enable */
 #define   CTRL_RUNLATCH	0x1
 #define SPRN_DAWR	0xB4
-#define SPRN_MPPR	0xB8	/* Micro Partition Prefetch Register */
 #define SPRN_RPR	0xBA	/* Relative Priority Register */
 #define SPRN_CIABR	0xBB
 #define   CIABR_PRIV		0x3

arch/powerpc/kvm/book3s_hv.c

@@ -36,7 +36,6 @@
 
 #include <asm/reg.h>
 #include <asm/cputable.h>
-#include <asm/cache.h>
 #include <asm/cacheflush.h>
 #include <asm/tlbflush.h>
 #include <asm/uaccess.h>
@@ -75,12 +74,6 @@
 
 static DECLARE_BITMAP(default_enabled_hcalls, MAX_HCALL_OPCODE/4 + 1);
 
-#if defined(CONFIG_PPC_64K_PAGES)
-#define MPP_BUFFER_ORDER	0
-#elif defined(CONFIG_PPC_4K_PAGES)
-#define MPP_BUFFER_ORDER	3
-#endif
-
 static int dynamic_mt_modes = 6;
 module_param(dynamic_mt_modes, int, S_IRUGO | S_IWUSR);
 MODULE_PARM_DESC(dynamic_mt_modes, "Set of allowed dynamic micro-threading modes: 0 (= none), 2, 4, or 6 (= 2 or 4)");
@@ -1455,13 +1448,6 @@ static struct kvmppc_vcore *kvmppc_vcore_create(struct kvm *kvm, int core)
 	vcore->kvm = kvm;
 	INIT_LIST_HEAD(&vcore->preempt_list);
 
-	vcore->mpp_buffer_is_valid = false;
-
-	if (cpu_has_feature(CPU_FTR_ARCH_207S))
-		vcore->mpp_buffer = (void *)__get_free_pages(
-			GFP_KERNEL|__GFP_ZERO,
-			MPP_BUFFER_ORDER);
-
 	return vcore;
 }
 
@@ -1894,33 +1880,6 @@ static int on_primary_thread(void)
 	return 1;
 }
 
-static void kvmppc_start_saving_l2_cache(struct kvmppc_vcore *vc)
-{
-	phys_addr_t phy_addr, mpp_addr;
-
-	phy_addr = (phys_addr_t)virt_to_phys(vc->mpp_buffer);
-	mpp_addr = phy_addr & PPC_MPPE_ADDRESS_MASK;
-
-	mtspr(SPRN_MPPR, mpp_addr | PPC_MPPR_FETCH_ABORT);
-	logmpp(mpp_addr | PPC_LOGMPP_LOG_L2);
-
-	vc->mpp_buffer_is_valid = true;
-}
-
-static void kvmppc_start_restoring_l2_cache(const struct kvmppc_vcore *vc)
-{
-	phys_addr_t phy_addr, mpp_addr;
-
-	phy_addr = virt_to_phys(vc->mpp_buffer);
-	mpp_addr = phy_addr & PPC_MPPE_ADDRESS_MASK;
-
-	/* We must abort any in-progress save operations to ensure
-	 * the table is valid so that prefetch engine knows when to
-	 * stop prefetching. */
-	logmpp(mpp_addr | PPC_LOGMPP_LOG_ABORT);
-	mtspr(SPRN_MPPR, mpp_addr | PPC_MPPR_FETCH_WHOLE_TABLE);
-}
-
 /*
  * A list of virtual cores for each physical CPU.
  * These are vcores that could run but their runner VCPU tasks are
@@ -2471,14 +2430,8 @@ static noinline void kvmppc_run_core(struct kvmppc_vcore *vc)
 
 	srcu_idx = srcu_read_lock(&vc->kvm->srcu);
 
-	if (vc->mpp_buffer_is_valid)
-		kvmppc_start_restoring_l2_cache(vc);
-
 	__kvmppc_vcore_entry();
 
-	if (vc->mpp_buffer)
-		kvmppc_start_saving_l2_cache(vc);
-
 	srcu_read_unlock(&vc->kvm->srcu, srcu_idx);
 
 	spin_lock(&vc->lock);
@@ -3073,14 +3026,8 @@ static void kvmppc_free_vcores(struct kvm *kvm)
 {
 	long int i;
 
-	for (i = 0; i < KVM_MAX_VCORES; ++i) {
-		if (kvm->arch.vcores[i] && kvm->arch.vcores[i]->mpp_buffer) {
-			struct kvmppc_vcore *vc = kvm->arch.vcores[i];
-			free_pages((unsigned long)vc->mpp_buffer,
-				   MPP_BUFFER_ORDER);
-		}
+	for (i = 0; i < KVM_MAX_VCORES; ++i)
 		kfree(kvm->arch.vcores[i]);
-	}
 	kvm->arch.online_vcores = 0;
 }